It is well established in the prior art that dissolved iron in the ferric oxidation state in aqueous acid solutions can lead to the formation of ferric iron-containing compounds in the bearing solution which produce insoluble iron solids when the pH of the acid solution increases to a value greater than approximately 4. In this regard, U.S. Pat. No. 4,683,954 to Walker and U.S. Pat. No. 5,084,192 to Dill teach that ferric compounds such as ferric hydroxide begin to precipitate from hydrochloric acid solution when the pH of the acid increases to a value of about 2.5 and greater and that precipitation is complete when the solution's pH is about 3.5. This precipitation phenomenon becomes a serious problem when an acid, such as hydrochloric acid, containing dissolved ferric iron is being used to react with a subsurface, acid soluble, calcareous formation, such as limestone, wherein the acid reaction causes the pH of the acid solution to typically spend to a value greater than the 4 and 5 range.
In addition to the precipitation problem discussed above which can be caused by the presence of ferric ion in acid, it is taught by several authorities that hydrochloric acid, particularly when at high concentrations of about 15% and greater, can cause the development of sludge when the acid is placed in contact with certain types of crude oil. The sludge formation problem is exacerbated when the acid which is in contact with the crude oil also contains ferric ion.
For purposes of this invention, sludge is defined as a solid material formed in crude oil containing asphaltenes and maltenes which constituents may, under certain conditions as pointed out above, precipitate from the crude oil. Sludge formed in crude oil while the crude oil is in a formation can render very difficult the task of recovery of the oil from the formation. Crude oil containing quantities of asphaltenes and maltenes subject to the production of sludge is referred to herein as sludging crude.
Accordingly, the sludging problem specifically addressed herein is caused by the combination of acid, especially high concentration hydrochloric acid, and ferric ion in contact with a sludging crude. This problem is particularly severe when the sludge is produced during formation acidizing.
Formation acidizing or simply, acidizing, is a well known method used to increase the flow of fluid from a subterranean formation. According to conventional practices, the underground formation is contacted with an acidic composition to react with and dissolve material contained therein for the purpose of increasing the permeability of the formation. The flow of fluid from the formation is therefore increased because of the increase in formation permeability caused by the dissolution of the material. A known method of acidizing comprises the steps of conducting an acid composition to the formation through tubing disposed in a borehole that penetrates the formation; forcing the acid composition into contact with the formation and permitting the acid to react with and dissolve certain materials contained therein to enlarge passages through the formation and thus increase the permeability of the medium so treated.
It is apparent that the object of formation acidizing, which is to increase formation permeability, can be frustrated if the very acid composition employed in the treatment to achieve that object produces an environment which fosters the development of solid material which can instead fill and plug pore spaces intended to be enlarged in the treated formation. Instead of enhancing fluid production, the consequent result is a failure to increase production, and even possibly decrease formation permeability.
In the context of formation acidizing, ferric ion can be introduced into the acid as a result of reaction between ferric compounds, such as rust and millscale, contacted in such locations as the tanks used to store the acid and/or transport it to a well that requires acidizing. Most common, such ferric compounds may be encountered on the interior walls of the conduit which conducts the acid down to the formation, reaction of the acid with ferrous compounds in the formation followed by oxidation of ferrous ion to ferric ion, and reaction between the acid and formation minerals that include ferric compounds, such as goethite, FeO(OH), magnetite, Fe.sub.3 O.sub.4, and hematite, Fe.sub.2 O.sub.3.
Solutions to the problems of precipitation of ferric iron compounds from spent acid and the formation of sludge induced by the contact between ferric ion and acid with sludging crude revolve about the control of ferric ion in the acid solutions and/or the elimination of those ions from the solution. One suggested mitigating procedure is the removal of ferric compounds from the metal conduits through which the acid solutions are conducted down-hole, such as by a process known as pickling, prior to the conduit's utilization in acidizing procedures.
Alternatively, the Dill patent ('192) discloses the use of a blend of formic acid and acetic acid, in combination with anti-sludge agents and iron control agents. U.S. Pat. No. 4,823,874 discloses the use of anti-sludging agents such as quaternary ammonium salts of fatty amines in hydrochloric acid. U.S. Pat. No. 4,574,050 to Crowe discloses the use of an iron control agent, such as ascorbic acid and erythorbic acid, in hydrochloric acid. U.S. Pat. No. 5,063,997 to Pachla appears to disclose the reduction of ferric ion to ferrous ion in hydrochloric acid with hypophosphorous acid and catalyst material selected from cupric and cuprous compounds.
In U.S. Pat. No. 5,445,221 to Vinson, the reduction of ferric ion to ferrous ion in hydrochloric acid is disclosed with certain sulfur-containing, non-ionic, organic compounds in combination with a separate catalyst material selected from copper and vanadium compounds. The disclosure of the '221 patent is detailed and accurate with respect to the background of that invention and the needs for ferric ion reduction for applications in oil field settings. For these same purposes of background information regarding the needs and applications for ferric ion and iron reducing capabilities, U.S. Pat. No. 5,445,221 is expressly incorporated herein by reference.
For purposes of contrast with the present invention, it should be appreciated that the reduction of ferric iron in the acidizing process of the '221 patent is accomplished through a mercaptan function wherein the sulfur containing, non-ionic organic compound reacts with the ferric ions to convert them to the more innocuous ferrous ions. Within the '221 patent it is specifically recognized that the mercaptan function alone (i.e. through the use of 2-mercaptoethanol exclusively) is only capable of reducing ferric ion to ferrous ion when in solution with organic acids and not in inorganic acids such as hydrochloric acid. Still further, it was appreciated in the '221 patent that utilization of the mercaptan function was only possible in lower concentrations of organic acids, those lower concentrations being at least less that 28%. The success of using the mercaptan function in acetic acid which is organic is demonstrated in EXAMPLE 8 of the '221 patent. EXAMPLE 13 of the '221 patent clearly indicates that the mercaptan function is ineffective for reducing ferric ions even in very low concentrations of inorganic acids. In that example, an acid mixture of 2% inorganic hydrochloric acid was prepared with a 10% organic acetic acid. With ferric ions present in the form of ferric chloride, the addition of 2-mercaptoethanol caused no color change and therefore no reduction of the ferric ions to the innocuous ferrous ions. Subsequently, cupric chloride was then added to that solution and a color change resulted indicative of the reduction of the ferric iron to ferrous iron. From this result, it was believed that the added cupric chloride acted as a catalyst to the sulfur containing 2-mercaptoethanol which in turn reduced the ferric iron to ferrous iron.
As will be discussed in greater detail hereinbelow, however, the present invention demonstrates that the copper containing cupric chloride was not merely the catalyst, but was instead the actual reducing agent. This finding is consistently supported in the examples of the '221 patent where either a copper containing compound or a vanadium containing compound was always added in combination with a mercaptan functioning compound; the only exception being EXAMPLE 8 which was conducted exclusively with an organic acetic acid, and not the more common and frequently used inorganic acids of which hydrochloric is an example.
In view of the obvious need developed above for ferric iron reducing compounds, the affected industries such as the petroleum industry and those providing support thereto have endeavored to develop a reducing agent formulation that can be readily prepared, transported, stored and ultimately utilized for its ferric iron reducing capabilities. During the development of the present invention, it has been appreciated that a substantial impediment to providing such a product for use at remote locations, such as at a well site, is the inability to prepare and maintain a homogenous mixture of the several components preferred in a ferric iron reducing additive. More specifically, it was observed that when the reducing agent and what had previously been believed to be a catalyst were mixed, constituent components precipitated from the solution and were difficult to redissolve. As a result, the standard practice prior to this invention's development has been to supply the several ingredient compounds unmixed to the end-user for combination at the point of application or use. Because the proportions of the several components can be critical, this was undesirable in that the ability to accurately measure and mix the components is difficult at best, and often not possible, especially on location. As a result, a primary objective of the present invention became the capability to prepare a ready-to-use ferric ion reducing additive that can be produced by the manufacturer and then shipped and stored for extended periods of time without separation of the constituent components.
In view of these objectives and in response to the industrial requirements for ferric iron reducing agents, the present invention was developed and through its development several discoveries were made with respect to the function of the different constituent components ultimately incorporated therein and the benefits that can be potentiated and derived therefrom.